As is disclosed in U.S. Pat. Nos. 3,765,258 and 3,942,390, there is known a transmission system for working vehicles comprising a pair of separate left and right hydrostatic transmissions which transmit power of an engine towards left and right final drive wheels (travelling wheels or caterpillar track-driving wheels) independently at continuously variable transmission ratios. In such transmission system, left and right speed-reduction mechanisms are interposed between the left and right hydrostatic transmissions and the left and right final drive wheels for transmitting rotation of output shafts of the hydrostatic transmissions to the final drive wheels with a reduced speed of rotation.
A vehicle including a transmission system of this type is steered or turned using a change lever device for controlling hydrostatic transmissions or variable displacement hydraulic pumps thereof. A spin turn of vehicle may be attained by controlling the left and right hydrostatic transmissions so as to drive the left and right final drive wheels to rotate into opposite directions with an equal speed of rotation. A gentle turn of vehicle with a desired turning radius may be attained by controlling the left and right hydrostatic transmissions so as to drive the left and right final drive wheels to rotate into a same direction with different speeds of rotation. It is thus seen that a transmission system comprising a pair of separate left and right hydrostatic transmissions provides an excellent maneuverability to a vehicle. Left and right speed-reduction mechanisms referred to before transmit the rotation of output shafts of left and right hydrostatic transmissions to axles of left and right final drive wheels with a reduced speed of rotation so as to enlarge the torque for driving the left and right travelling wheels or track-driving wheels. Such speed-reduction mechanisms will also eliminate, when a vehicle is travelled with a low speed, a necessity of controlling the vehicle speed by bringing hydrostatic transmissions into a low speed range where output rotation speed of the hydrostatic transmissions becomes unstable.
Two typical examples of the structure according to the prior art are shown in the two U.S. patents referred to before.
In the transmission system shown in U.S. Pat. No. 3,765,258, an engine is mounted on a front portion of vehicle in a posture slightly inclined towards a forward and downward direction. Forwardly of this engine and in the middle of the vehicle is arranged a gear box. Left and right hydrostatic transmissions are mounted on the left and right side walls of such gear box into which input shafts as well as output shafts of the left and right hydrostatic transmissions extend. Power of the engine is transmitted into the gear box and then to the input shafts via a bevel gearing. Axles of left and right track-driving wheels are arranged so that they extend from a low level within the gear box largely towards the left and right, respectively. Left and right speed-reduction mechanisms are disposed within the gear box and are composed of left and right gear trains each having a smaller spur gear on each of the output shafts and a larger spur gear on each of the axles.
In the transmission system shown in U.S. Pat. No. 3,942,930, while an engine is mounted on a front portion of vehicle, separate left and right hydrostatic transmissions are arranged on a rear portion of the vehicle in a horizontal posture such that input shafts thereof extend forwardly whereas output shafts extend rearwardly. A horizontal transmission shaft, drivenly connected to the engine output shaft, is provided which passes between the left and right hydrostatic transmissions and extends to a rear end portion of the vehicle so as to be utilized as a power takeoff shaft for a working implement. Input shafts of the left and right hydrostatic transmissions are drivenly connected to such transmission shaft via left and right transmission devices having belts, respectively. Left and right final drive units which comprise as output shafts thereof axles of left and right track-driving wheels are disposed on the underside of vehicle and behind the left and right hydrostatic transmissions. Such left and right drive units comprise forwardly extending left and right input shafts between which a horizontal cluster-sprocket support shaft is disposed. Left and right speed-reduction mechanisms are provided each of which comprises first and second speed-reducing trains having first and second chains. The first chain is entrained over a first sprocket, fixedly mounted on the output shaft of each hydrostatic transmission, and a second sprocket rotatably mounted on the sprocket support shaft. The second chain is entrained over a third sprocket, rotatably mounted on the support shaft and rotatable in unison with the second sprocket, and a fourth sprocket fixedly mounted on the input shaft of each drive unit.
As described above, the transmission system disclosed in U.S. Pat. No. 3,942,390 is fashioned such that the hydrostatic transmission and final drive unit of each of the left and right sides are provided separately from each other and are placed at different locations. Between such hydrostatic transmission and final drive unit are disposed two trains of chains. It is thus seen that the transmission system occupies a considerable space as a whole and the assemblage thereof is troublesome. Contrarily to this, the transmission system disclosed in U.S. Pat. No. 3,765,258 occupies a relatively small space and can be assembled with ease. This is because the left and right hydrostatic transmissions employed in this system are mounted on the outer surfaces of a gear box including a bevel gearing, by which a transmission path from engine is bent and is connected to the input shafts of left and right hydrostatic transmissions, and left and right reduction gear trains which are disposed between the output shafts of left and right hydrostatic transmissions and the axles of left and right track-driving wheels.
The transmission system disclosed in U.S. Pat. No. 3,765,258 includes, however, only one reduction gear train in each of the transmission paths between the left and right hydrostatic transmissions and the left and right track-driving wheels and is thus inferior to the transmission system disclosed in U.S. Pat. No. 3,942,390 in an ability of enlarging the vehicle-driving torque and also in securing a stable vehicle speed at a low speed range. It is considered that, if a further reduction gear train were additionally provided at each side of the interior of the gear box disclosed in U.S. Pat. No. 3,765,258, then it would be required to enlarge such gear box considerably either in length or in height. Enlargement of the gear box in length will result in an enlargement of the length of vehicle. Enlargement of the gear box in height will cause an undue lowering of the level of final drive wheels and axles thereof so that a stable support of vehicle body by caterpillar tracks or travelling wheels is damaged.
In a transmission system comprising left and right wheel axles which extend laterally outwardly from a gear box having a small width and arranged in the middle of the vehicle, such as the one disclosed in U.S. Pat. No. 3,765,258, a large interval is caused between each caterpillar track or travelling wheel, which supports the vehicle on the ground, and the heavy gear box and hydrostatic transmissions so that a large stiffness is required to the vehicle body or frame. Furthermore a concentrated arrangement of transmission units in the middle of width of the vehicle may provide a restriction to the arrangement of a power take-off mechanism which is often fashioned to have a transmission path extending through such middle portion of the vehicle.